A significant part of the assessment of risk is based on the exposure of sensitive populations to environmental contaminants. It has become clear over the last several years that assessments of environmental concentrations of many hydrophobic organic contaminants (HOC) based on total amount extractable may vastly over-estimate the exposure concentration. A large fraction of HOC may be sequestered in forms that are not readily bioavailable. The amount so sequestered appear to increase over time or during aging in the soil or sediment. There have been several proposed mechanisms to account for this loss of bioavailability, including sorption onto or partitioning into the soil organic carbon, physical entrapment within the organic matter or soil pores, organic binding of the HOC or their microbial degradation products within the soil organic carbon matrix or an absence of factors, such as adequate electron acceptors that will make the sequestered materials degradable. We have also discovered that the presence of active microbes promotes sequestration. Understanding the mechanism is critical if we are to adequately describe the health risk of these chemicals. The current state of knowledge does not permit the sequestration mechanisms to be identified and therefore the risk of the HOC reappearing is unknown. The specific aims of the proposed research are to: 1. Investigate the factors that control the mechanisms of sequestration including properties of the mineral and organic fractions of soil. This includes examination of the role of microbes both in changing the soil organic matter and the HOC. 2. Examine the role of redox changes and added electron acceptors on the transformations and sequestration of HOC in soil. 3. Expand the soils and chemicals studies to include soils that have been aged for nearly 50 years with a variety of chlorinated hydrocarbon insecticides to determine whether mechanisms identified in Aim 1 for PAH are also operative for other soils and chemicals. 4. Investigate how manipulation of mechanism regulating factors can impact bioavailability and serve as the basis for Bioremediation strategies.